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Four-way cosmic pile up

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 16 April, 2009

Combining images from space- and ground-based telescopes, astronomers have revealed the first cosmic collision of four separate galaxy clusters.

The image was acquired using data from NASA’s Chandra X-ray Observatory, Hubble Space Telescope and the Keck Observatory in Hawaii, allowing astronomers to determine the three-dimensional geometry and motion of the galaxies in the system MACSJ0717.5+3745 (or MACSJ0717 for short). The resulting image captured a messy four-way pile up resulting from a 13 million light year long stream of galaxies and dark matter pouring into a region already containing a significant number of galactic inhabitants.

This composite image shows the massive galaxy cluster MACS J0717 where four separate galaxy clusters have been involved in a collision. Hot gas is shown in an image from NASA's Chandra X-ray Observatory, and galaxies are shown in an optical image from NASA's Hubble Space Telescope. The hot gas is colour-coded to show temperature, where the coolest gas is reddish purple, the hottest gas is blue, and the temperatures in between are purple. Image: NASA, ESA, CXC, C. Ma, H. Ebeling, and E. Barrett (University of Hawaii/IfA), et al., and STScI.

Lead author of the study Cheng-Jiun Ma of the University of Hawaii comments that the system also displays a remarkably hot temperature. “Since each of these collisions releases energy in the form of heat, MACSJ0717 has one of the highest temperatures ever seen in such a system.”

Previous images had already revealed the presence of the filament, but this is the first time that evidence holding it responsible for the mayhem has been brought to light. And the evidence is strong. First, the motion of gas and galaxy clusters matched the direction and orientation of the filament, and second, the largest hot region in MACSJ0717 is where the filament intersects the cluster, suggesting ongoing impacts.

“MACSJ0717 shows how giant galaxy clusters interact with their environment on scales of many millions of light years,” says team member Harald Ebeling. “This is a wonderful system for studying how clusters grow as material falls into them along filaments.”

The observations match well to computer models that show that massive galaxy clusters should grow in regions where large-scale filaments of intergalactic gas, galaxies, and dark matter intersect, and where material falls inward along the filaments. But there is still a lot to learn, and Ma and his team hope to use even deeper X-ray data to measure the temperature of the gas over the full 13 million light year extent of the filament, to see how its infall may heat the gas in clusters over large scales.

“This is the most spectacular and most disturbed cluster I have ever seen,” says Ma, “and we think that we can learn a whole lot more from it about how structure in our Universe grows and evolves.”

MACS J0717 is located 5.4 billion light years from Earth.

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